Firefox <strike>52</strike> 57 and later releases only support WebExtensions and disable all Legacy extensions If you need to use Legacy extensions then you can use the Firefox 52 ESR release. Otherwise you can stay with the current Firefox release version.

• https://support.mozilla.org/en-US/kb/switch-to-firefox-extended-support-release-esr

Firefox 52.5.3 ESR:

• https://www.mozilla.org/en-US/firefox/organizations/all/
• https://www.mozilla.org/en-US/firefox/52.5.3/releasenotes/
• https://www.mozilla.org/en-US/firefox/52.5.3/system-requirements/

It seems the owner of the Property Bee extension has no plans to update it for Firefox 57: http://www.property-bee.com/forum/viewtopic.php?p=207687#p207687

(Please ignore their suggestion to use an unofficial download site for older versions of Firefox. See cor-el's links or Install an older version of Firefox.)

That extension sends requests to a server for information about the properties listed on a page. It seems that could be implemented as a compatible extension, but I don't know who has the motivation to do the work...

This new extension could be similar, but it's hard for me to tell and there aren't any reviews yet: https://addons.mozilla.org/firefox/addon/asking-price-revealer/